The present invention relates to an automatic equalizer apparatus, and more particularly to an automatic equalizer apparatus used in performing data transmission through telephone lines, for causing a modulator and demodulator (that is, a MODEM) to perform automatic phase equalization well adapted to frequency characteristics of carrier-band lines, through which a transmission signal has passed.
When a data signal is transmitted through telephone lines, the receiving equipment is required to have an equalizer to compensate for variations in a transmission characteristic between data terminal equipment. Especially, in data transmission, it is impossible to carry out high-quality data communication unless a group delay distortion at (that is, a phase characteristic of) a transmission line is equalized accurately. Accordingly, a MODEM in the receiving equipment is usually provided with an automatic equalizer.
However, a conventional automatic equalizer is limited in equalizing ability, and can operate satisfactorily only in the case where the number of carrier-band lines (in another word, the number of links) is smaller than three or four. When the number of carrier-band lines is large, it arouses a problem in the structure of the automatic equalizer to perform phase equalization only by the conventional automatic equalizer. In more detail, although various phase equalizers have been known, in order to reduce the size and digitize these phase equalizers, it is desirable to form the so-called transversal filter, in which input and output signals of a number of delay elements are multiplied by predetermined coefficients and the signals thus multiplied are added to each other, of a digital signal processor. However, in the case where equalization has to be made in a wide range, the number of multiplications is large, and it is impossible to process such multiplications with a digital signal processor.
In order to solve this problem, a method has been used in which there is prepared a fixed equalizer capable of compensating for variations in transmission characteristic when the number of links is smaller than about three, the number of links is checked at the starting time of data transmission, and the fixed equalizer is manually inserted into a MODEM when it is judged that the number of links is greater than or equal to a predetermined number (for example, four).
In the above-mentioned method, however, the number of links is detected in a complicated manner. Further, the fixed equalizer has to be manually inserted in the MODEM when necessary. Accordingly, in the case where exchange of data transmission is frequently performed, this method is very disadvantageous to the effective use of data transmission line since it takes much time to detect the number of the links and to adapt the equalizer to the characteristic of the links.
Further, another method has been proposed in which the number of links is detected and an optimum equalizer is formed in accordance with the number of links. However, a special signal for detecting the number of links is required in this method. Further, as mentioned previously, it is impossible to form the optimum equalizer of a digital signal processor.